Generally, in a swash plate type piston pump, a back surface (convex surface) of a swash plate projects in a circular-arc shape, a circular-arc supporting surface (concave surface) is formed on a swash plate supporting portion, and the projecting circular-arc back surface of the swash plate is supported by the supporting surface so as to be able to tilt. A tilt angle of the swash plate with respect to a rotating shaft can be changed by tilting the swash plate. With this, the amount of discharged hydraulic oil can be adjusted (see Patent Document 1 for example).
Specifically, this piston pump is configured such that a plurality of pistons are included in a cylinder block provided in a casing to be arranged in a circumferential direction, and the cylinder block rotates in accordance with the rotation of the rotating shaft. By the rotation of the cylinder block, the piston reciprocates while a tip end portion thereof is guided along the swash plate. Thus, the piston can suck and discharge the hydraulic oil. Here, if the tilt angle of the swash plate is increased, a stroke of the piston increases, and this increases the amount of discharged hydraulic oil. In contrast, if the tilt angle is decreased, the stroke of the piston decreases, and this decreases the amount of discharged hydraulic oil.
In order to increase or decrease the tilt angle of the swash plate, a tilt adjustment driving portion is provided. The tilt adjustment driving portion includes a tilt adjustment cylinder and a tilt adjustment piston configured to slide in the tilt adjustment cylinder to change the tilt angle of the swash plate.
The tilt adjustment driving portion can change the position of the tilt adjustment piston in response to a control command from a mounting apparatus to change the tilt angle of the swash plate. Therefore, during the operation of the swash plate type piston pump, the tilt adjustment piston slides back and forth at all times in order to control the amount of discharged hydraulic oil at all times in accordance with, for example, the amount of hydraulic oil used by the apparatus. Similarly, during the operation of the swash plate type piston pump as a motor, the tilt adjustment piston slides back and forth at all times in order for the number of rotations of the rotating shaft to be controlled to the number changed in response to the command from the mounting apparatus, for example.
Depending on a positional relation between the tilt adjustment piston of the tilt adjustment driving portion and the swash plate, a component force (lateral component force) may be applied to the tilt adjustment piston in a direction perpendicular to an axial direction of the tilt adjustment piston. With this, the tilt adjustment piston may slide back and forth while applying a high surface pressure to an inner surface of the tilt adjustment cylinder. In this case, a lubricating oil film at an interface between the tilt adjustment cylinder and the tilt adjustment piston tends to be cut. Therefore, each of a sliding surface of the tilt adjustment cylinder and a sliding surface of the tilt adjustment piston requires seizing resistance and abrasion resistance.
Conventionally, the tilt adjustment cylinder made of cast iron is subjected to gas nitrocarburizing for hardening the surface thereof by causing nitrogen to diffusively intrude or infiltrate into the cast iron. Thus, the seizing resistance and the abrasion resistance are given to the tilt adjustment cylinder.    Patent Document 1: Japanese Laid-Open Patent Application Publication No. 11-50951